In interline transfer type imaging devices, photogenerated charge is collected on a pn junction or under the gate of a photocapacitor for a period of time and then transferred into a charge coupled register to be detected by an output circuit. During the time required for the read-out operation, light is still incident on the photodiode or photocapacitor, and photogenerated charge is being collected and stored for the following frame. In this mode of operation, it is necessary to shield the shift registers and transfer gate regions from exposure to the incident light. Such exposure will produce spurious signals in the detected image, thus degrading the performance of the device. This unwanted exposure has been called "image smear" in the literature.
A variety of causes for image smear have been discussed and reviewed in a recent paper by Teranishi and Ishihara, IEEE Transactions on Electron Devices, ED-34, 1052, (1987). A significant cause of smear identified by these authors is due to a waveguide effect which occurs between the light shield layer of the device and the underlying polysilicon CCD gates. This effect is illustrated in FIG. 1a.
In FIG. 1a, a semiconductor substrate 10 of first conductivity type, typically n, contains a doped region 20 of a second, opposite, conductivity type, typically p, a barrier region 30 consisting of higher doping second type conductivity and thick insulator 60, a photodiode region 40 of first conductivity type, a transfer channel region 50 of first conductivity type, a charge transfer gate 70, an insulating layer 75 over both the transfer gate and the photodiode, a smoothing layer of insulator 80, opaque light shield layer 90 and a top protective insulator layer 100. Incident light rays 110 and 112, illustrated schematically, are able to penetrate into the transfer channel region of the device where electron-hole pairs may be produced. Charges so produced may be collected in the transfer channel 50 and read out along with the desired photogenerated charge which has been collected by the photodiode 40. This charge is unwanted and produces smear.
As Teranishi and Ishihara have noted, it is desirable to have thinner insulating layers between the light shield and the underlying semiconductor device in order to minimize the effects of light rays such as 110 and 112. Typically, a layer of phosphosilicate glass is used for this insulator, and the light shield is aluminum. Teranishi and Ishihara minimized the insulator thickness by only using a thermally grown oxide layer for the insulator. This is illustrated in FIG. 2 where an aluminum light shield 95 is separated from the gate electrode and photodiode by thermally grown oxide 75. In devices such as shown by Teranishi et al, the aluminum light shield 95 can be subject to hillock growth causing severe topographic features.
In image sensors for detecting color, however, a color filter pattern must still be fabricated over the light shield. The structure of FIG. 1, due to the topography of the surface, is not well suited for color filter application, and additional smoothing layers must be applied before applying the color filter array. Such smoothing layers are typically rather thick layers of spun-on organic materials which can present manufacturing problems due the thickness required, and can add to image smear and "color cross-talk" as described by McColgin and Pace, U.S. Pat. No. 4,553,153. The FIG. 1b surface has even more severe topographical features than the FIG. 1a structure.
In Japanese Published Application No. 57-24171 to Miyata dated Feb. 8, 1982, it has been disclosed to provide a refractory light shield made of a metal or its silicide. WSi.sub.2 is one of the listed silicides. WSi.sub.2 has good advantages, however, it produces a light shield with inadequate opacity at 3000 A or less. In other words, it permits too much light to pass into the substrate.